Communication links, such as optical links, herein defined as optical links for carrying optical signals containing e.g. data—thus providing an optical data link such as the Ethernet standard links—or other types of information, are provided by one or more transmitters, often laser transmitters, cabling in the form of one or more optical fibers, such as Multi-Mode optical Fibers (MMF) and one or more receivers, and it is convenient within the field to specify these separately and standardize these to allow interoperability. Further, for standardization purposes within the transmitter manufacturing industry it is convenient to specify the transmitter (Tx) by reference to a small number of metrics that correlate to expected performance when used with the worst allowed optical fiber and receiver (Rx). The general term “Transmitter and Dispersion Penalty” has been used as a primary metric for providing a quantified measure of a transmitter's quality and can be used in a specification that allows a trade-off between quality and the transmitted signal amplitude. Many versions of TDP have been defined, and are generally related for providing a particular bit error ratio (BER), as will be further described below. It has been convenient for the standards to measure the transmitter close to its output, at TP2, see FIG. 3, being generally in the field considered to be not the output of the transmitter itself, but instead the output of a patch cord (indicated with an ellipse on the line), without using a worst-case optical fiber and receiver (this is done in TDP for multimode fiber), and also without a reference transmitter.
The eye mask definitions or “eye” is well known but do not provide good correlation to expected performance when used with the worst allowed optical fiber and receiver.
As mentioned above, see FIG. 3, the transmitter, optical fiber, and receiver impair and attenuate the signal, and when the optical fiber is a MMF, filtering and distortion is primarily caused by modal dispersion and chromatic dispersion. If the oscilloscope is provided at TP2, i.e. after the patch cord and not linked to a longer length optical fiber for example, the eye-mask at TP2 cannot correlate well to link performance, because the optical fiber and the receiver itself change or attenuate the signal and cause noise therein.
Other considerations for increasing signaling rates are noise arising in the elements of the link (in and between transmitter, optical fiber and receiver, respectively) and the instrument noise in a measurement. Forward error correction (FEC) may be used, in which case the required BER for the link, before correction, can be e.g. 5×10−5 for arriving at a corrected BER of 10−12. The relaxed BER means that fewer samples are needed in a measurement so an oscilloscope may be used rather than a BERT, i.e. bit error ratio tester.
At the Institute of Electrical and Electronics Engineers (IEEE) 802.3bm standard group, called the 40 Gb/s and 100 Gb/s (Gigabit per second) Fiber Optic Task Force, it was discussed whether to switch the IEEE transmitter specification for 100 Gb/s Ethernet optical links of the port type 100GBASE-SR4, from the present industry standard method in a similar way to the general term named the “Transmitter and Dispersion Penalty” (TDP) to a different oscilloscope or scope based method, because the established TDP method is thought to be too difficult to do accurately in practice, and the ability of TDP to adequately predict link margin for MMF links is questioned.
The 100GBASE-SR4 is a port type for multi-mode optical fiber defined in IEEE Task Force 802.3bm and uses 850 nm lasers for the optical transmitters Tx. Its physical coding sublayer 64b/66b PCS is defined in IEEE 802.3 Clause 82, its FEC transcoding in Clause 91 and its Physical Medium Dependent PMD in Clause 95. It uses four lanes of multi-mode fiber delivering serialized RS-FEC encoded data at a rate of 25.78125 Gb/s per lane. An 8-fiber link providing four 25 Gb/s lanes in each direction, up to 70-100 m long, uses Optical Multimode (OM) MMF, such as the OM3 and/or OM4 type MMF. The groups of 802.3 are generally concerned with the maintenance and extension of the Ethernet data communications standard. Thus, 100GBASE-SR4 is one of the latest Physical Layer or PHY standards of the IEEE 802.3 Ethernet Working Group. 100GBASE-SR4 is primarily used in datacenter storage servers and high-performance servers, and in Ethernet switches.
TDP is a known standard method for measuring penalty which is used to control the Bit Error Ratio (BER) in the conveyed optical signal in particular by the transmitter Tx and through the optical fiber, i.e. part of the optical data link (ODL), which should be lower than 1E-12 (1 per 1012 bits) required today for most data communication systems. TDP is the difference in sensitivity for a reference receiver when comparing an ideal transmitter with a very short fiber against the transmitter under test with the rated fiber dispersion. It is defined by the change in receiver sensitivity due to transmitter impairments and its transmission over a defined optical path. It is manifested as a shift of the system's Bit Error Ratio (BER) curves for these two cases: 1) The reference condition: ideal transmitter (Tx) (specified with minimal rise and fall times and noise) with the same Optical Modulation Amplitude (OMA) as the actual Device Under Test (DUT) with no dispersive fiber in the path and the standard reference receiver bandwidth; 2) The impaired condition: DUT with specified fiber path and/or a lower reference receiver bandwidth. TDP is the ratio, or difference in decibels, between the received OMAs measured at the specified BER (sensitivities) in these two scenarios. When used for multimode link specification, the low bandwidth of a specified fiber path is implemented as a filter in the reference receiver in order to give consistent and accurate measurements. This parameter TDP is defined in IEEE 802.3ae-2002, for 10 Gb/s Ethernet and a similar definition was proposed in the draft amendment IEEE, P802.3bm.
TDP enables a trade-off between signal strength and signal quality. TDP is further suitable both for links that are protected by Forward Error Correcting (FEC) and for links which are not.
The reason why it was discussed to switch was that the known TDP used for 100GBASE-SR4 was thought to be too difficult to do in practice because it required using a reference transmitter (Tx ref) and special reference receiver (Rx ref). These are believed not to be generally commercially available, and building them from available parts is time consuming and not up to standards. Both the reference transmitter and the special reference receiver would need calibrating and this would add expenses to be avoided.
Alternative methods for estimating or calculating TDP's for the 100GBASE-SR4 have been suggested, but still a satisfactory one is needed in the field with the purpose of estimating how well the transmitter will perform within a given optical link, such as a MMF or a worst MMF link specified by 100GBASE-SR4.
One such alternative method, called VECPq has been proposed for Fiber Channel which is another optical communication standard. In this method, one measures an averaged signal with a Pseudo Random Binary Sequence (PRBS) test pattern with generator of length 9 (PRBS9). This has the advantage that no special reference transmitter or receiver is needed, and it calculates the averaged signal in the right bandwidth assuming the worst case link. Some disadvantages are that it does not screen for problems that arise with a longer pattern or service signal, and does not give a transmitter any credit for having better-than-worst noise. VECPq is actually a measure of the Signal to Noise Ratio (SNR) margin and not of penalty and does not treat different transmitters with the same penalty equally. Therefore, it is not always suitable for a trade-off between signal strength and signal quality. If a VECPq limit is set taking these weaknesses into account, VECPq as a standard may be hard for typical laser transmitters to meet up to.
Another such alternative, called VECP or TxVEC, is quite adequate for showing that a very good transmitter is actually very good, but may not be useful for establishing whether a mediocre transmitter is adequate. VECP is neither a measure of the SNR margin nor of penalty and does not treat different transmitters with the same penalty equally. Further, it measures the signal in the wrong bandwidth, i.e. not as it will be used in the worst case link scenario. Therefore, it is not suitable for a trade-off between signal strength and signal quality. If a VECP limit is set taking these weaknesses into account, VECP as a standard may also be very hard for typical lasers to meet up to.
An alternative was adopted by the Optical Internetworking Forum (OIF) CEI-28G-VSR “Common Electrical I/O (CEI)—Electrical and Jitter Interoperability agreements for 6G+ bps, 11G+ bps and 25G+ bps I/O clause 13”. This method uses extrapolation to find the desired percentile. However, this is an electrical specification and not optical; the expected variety of transmitters is much less, and although correlation to actual penalty after an electrical channel is poor, buying out the uncertainty with signal strength is affordable in this case, and a trade-off between signal strength and signal quality was not envisaged to be necessary.
As mentioned above, a known technique within the field is using the “eye” i.e. eye mask test, which is generally known to the skilled person as a basic test of transmitter performance. The eye diagram is provided using an oscilloscope receiver's display in which a pseudo-random digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the signalling rate is used to trigger the horizontal sweep. System performance information can then be derived by analyzing the display. A more open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to inter-symbol interference and noise appears as closure of the eye pattern. Histograms can be provided, showing the signal reception density statistics at any one particular part of the eye diagram.
The advantages here of the eye mask test are that no special reference transmitter Tref or additional receiver is needed, it is familiar to the skilled person, it screens for problems that arise with a longer pattern or service signal, and gives a transmitter credit for having better-than-worst noise. However, the disadvantages are that it does not measure a penalty or an SNR margin and does not treat different transmitters with the same penalty equally. Further, it may measure the signal in the wrong bandwidth, i.e. not as it will be used in the worst case link scenario. Poor correlation to actual penalty after a fiber means that the specification would have to be set very harsh to avoid unusable transmitters passing. A lack of consensus in defining mask margin would have to be overcome to enable trade-off between signal strength and signal quality. The observed eye includes noise from the oscilloscope, which is significant at the bandwidth needed for 100GBASE-SR4, and it is difficult to correct the eye measurement for this noise.
Another known method is to generate a so-called “bathtub curve” in which the decision point of a receiver is scanned across the eye, and the BER at each decision point is found. However, the bathtub of the transmitted signal does not directly correlate to the useful performance in the complete link with worst-case optical fiber and receiver, and the bathtub is affected by the noise of the test receiver. Single points from bathtub curves do not accurately predict performance after a worst case fiber and receiver.